Abstract An estimated 3 million Americans have chronic Hepatitis C (HCV) infection, and of those, up to 25% are co- infected with HIV. Of the new HCV infections, 80% occur in people who inject drugs (PWID) and successful treatment strategies for this core group are required to break this epidemic. Directly acting antiviral agents (DAAs) are new non-immune based therapies to treat HCV and are effective in treating HCV with HIV co-infection; still individuals with co-infection have more rapid progression of hepatic fibrosis and worse clinical outcomes than those with HCV mono-infection. PWID with HIV infection have significantly higher rates of re-infection and liver disease progression even after achieving sustained virologic response (SVR). It is critical to investigate the long- term durability of SVR in these high-risk populations in order to identify probable factors that contribute to HCV recurrence. Our group has been studying HCV treatment responses (including HIV positive PWID patients) in Baltimore and District of Columbia. We showed that in patients achieving SVR with DAA therapy, HCV specific T cell immunity is recovered in contrast to those with relapse of HCV. We are seeking to identify immune correlates of successful treatment response, especially in high-risk groups, with the goal of identifying persistent defects in virus specific immune recovery that may hamper durability of SVR. In this proposal, we will seek to 1) investigate the improvement in liver disease with SVR and HCV re-infection rates post SVR in HIV/HCV co- infected patients with or without current injection drug use 2) explore the impact of HIV/HCV co-infection and current injection drug use on recovery and maintenance of HCV-specific peripheral CD4 and CD8 T- cell responses after DAA therapy and 3) characterize the persistence of hepatic immune defects after SVR in HIV/HCV co-infected high-risk patient groups. Through these specific aims, we will elucidate the immune mechanisms and factors that drive re-infection in HIV/HCV co-infected PWID. We hypothesize that DAA-based eradication of HCV will result in augmentation of virus specific adaptive immunity, the lack of which may be associated with adverse outcomes in HIV/HCV co-infected patients and current PWID. We will employ standard and novel research techniques, including measuring soluble and cellular markers of inflammation and immune activation, conventional flow cytometry and novel mass cytometry for immunophenotyping diverse lymphocyte populations in periphery and liver and hepatic transcriptome sequencing to quantify HCV-specific immunity in these patients. Our proposal will provide valuable insights in protective immunity against HCV in HIV/HCV co- infected PWID who are at highest risk of reinfection and disease progression and are at the core of HCV epidemic. This will help us identify patients that may require additional intervention, and pave the way for development of innovative preventive strategies targeting HIV infected PWID enabling us to disrupt the ongoing HCV epidemic.